I’m not sure I understand what you mean when you ask about format. Let me tell you what I do; that may answer your question.

When I want referrals I just ask the client or the editor for them.

If I’m on the phone I might say something like “who else should I be talking with about writing this sort of thing?” Often I’ll get two or three names and contacts right then.

Another way is by email. I usually wait until we’re at the end of what we’re doing – or almost. Assuming they say that they are happy with what I’ve done, I may first ask if I can use that email as a testimonial, and then ask they who they know who might be interested in the same kind of writing, or in having me write for them.

Some successful writers I know send their ex clients some sort of a newsletter once or twice a year. The idea, of course, is to let people know you’re still in business and to ask for repeat business and referrals. Some are bold enough to ask directly; others do it more subtly and by implication.

My experience tells me that most people are delighted to help – you only have to ask them.

As you know it’s an issue that many professionals face, and don’t like. For some, asking for a referral is akin to begging for business. Of course, it need not be like that, and your post makes this clear.